Fundamentally, pressure sensitive copying paper consists of two sheets of paper, the lower surface of the upper leaf having applied thereto minute capsules containing a color reactive achromatic substance (hereinafter called "a color former") dissolved in a low volatile solvent and the upper surface of the lower leaf being coated with a solid acid, such as acid clay, phenolic compounds, etc. When the lower surface of the upper leaf and the upper surface of the lower leaf are put together and pressure is applied to the upper surface of the upper leaf by handwriting or typing, the capsules under pressure are broken and the color former contained therein is absorbed by the solid acid on the upper surface of the lower leaf to develop a color, thus providing a copy. There is also a type of pressure sensitive copying paper having a mechanism to develop color when a regional pressure is exerted upon a sheet of paper holding capsules, which contain a color former, and a solid acid on or within it. However, such type is quite similar to the first mentioned pressure sensitive copying paper consisting of two sheets of paper, an upper and lower leaf with respect to the mechanism of color formation, and thus the same color former can be used for both types.
As the color former, various kinds of chemical compounds have been used, and, among others, the fluoran compounds have recently been occupying an important position in the pressure sensitive copying paper industry because of the rich variety of colors that such compounds develop and the high color-developing speeds.
For example, 3,6-dimethoxy fluoran (Beilstein Hauptwerk Vol. XIX, p.225) readily develops into yellow; 7-chloro-6-methyl-3-diethylamino fluoran (Japanese Pat. Publ'n. No. 21,199/1968) into yellowish red; rhodamine lactone (Beilstein Hauptwerk Vol. XIX, p. 344) into bluish red; and 3-diethylamino-7-methylamino fluoran (Canadian Pat. No. 814,948) into green-blackish green. Among the above-mentioned derivatives and homologues, many of them have become commercially available as dyes for pressure sensitive copying paper.